fix(api): include exhaustion reason in "Node exhausted" placement warning

[AdaAibaby]

Summary

Closes #3278

The "Node exhausted, trying another node" Warn log in PlaceSandbox discarded the gRPC error from the orchestrator, making it impossible to tell from API logs which resource limit was hit.

Change

One-line fix in packages/api/internal/orchestrator/placement/placement.go:

// Before
logger.L().Warn(ctx, "Node exhausted, trying another node",
    logger.WithSandboxID(...), logger.WithNodeID(failedNode.ID))

// After
logger.L().Warn(ctx, "Node exhausted, trying another node",
    logger.WithSandboxID(...), logger.WithNodeID(failedNode.ID),
    zap.Error(utils.UnwrapGRPCError(err)))

This matches the pattern already used in the default branch two lines below.

Before / After

Before:

WARN  Node exhausted, trying another node  {"sandbox.id": "i5ksfunc...", "node.id": "cpu-nat-398"}

After:

WARN  Node exhausted, trying another node  {"sandbox.id": "i5ksfunc...", "node.id": "cpu-nat-398", "error": "too many sandboxes starting on this node, please retry"}

or

WARN  Node exhausted, trying another node  {"sandbox.id": "i5ksfunc...", "node.id": "cpu-nat-398", "error": "max number of running sandboxes on node reached (350), please retry"}

Now you can tell at a glance whether the node hit its concurrent-start limit (MaxStartingInstancesPerNode, default 3) or its total-sandbox limit (MaxSandboxesPerNode).

Test plan

• Trigger sandbox creation against a node at capacity; confirm "Node exhausted" Warn log now includes the error field with the specific reason

[gemini-code-assist]

Code Review

This pull request adds error logging to the sandbox placement warning when a node is exhausted by using utils.UnwrapGRPCError(err). Feedback indicates that utils.UnwrapGRPCError uses github.com/gogo/status instead of the standard google.golang.org/grpc/status, which prevents standard gRPC errors from being unwrapped correctly. It is recommended to update the helper function to use the standard gRPC status package.

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